The problem of inhomogeneity of liquids pervades many industries, such as paint, plastics, petroleum, glass, foods, pharmaceuticals and medical. In order to choose the most effective means of of mixing it is helpful to understand the scale and nature of the inhomogeneity. Liquids dissolving solids is a common situation. When one or both of the constituents are viscous the rate of solution can be slow. Heat increases that rate simply by reducing the viscosity. Mechanical mixing can increase that rate still more. Mechanical mixing is typically on a gross scale however when applied in certain ways known as high shear mixing, homogeneity on a microscopic scale can also be enhanced. Concrete as well as certain foods are mixed on a coarse scale. Paint, plastics and most foods must be homogeneous on a macroscopic scale which requires blending or ultrasonic mixing. Some plastics, pharmaceuticals and particularly optical glass become homogeneous on the molecular or atomic scale only with time for diffusion, a phenomena which is quite temperature dependent.
Glass must be homogeneous at all of these levels, that is, the molecular, microscopic, macroscopic and bulk scales; dimensionally this means from a few tens of angstroms through the micrometer range and larger. Homogeneity in glass is accomplished in three general ways; first the constituent materials are usually dry mixed by blending and often by also milling, secondly, after mixing during melting by convection stirring sometimes enhanced by various means of agitation, and thirdly by diffusion.
The properties of different types of glass are obtained by the selection and proportions of the various metallic oxides used. Furnace lining materials normally are also a mixture of metallic oxides. In spite of the fact that the lining composition is chosen so as to minimize its solubility in the glass, some erosion occurs anyway. The solution of the lining changes the glass composition and properties, creating inhomogeniety, stones and cords and eventually destroys the lining. As the fining temperature is increased the glass viscosity decreases thereby allowing bubbles to rise faster and chemical reactions to proceed faster to allow the fining time to be reduced. Unfortunately the increased temperature also increases the erosion rate of the furnace lining to increase inhomogeniety and reduce furnace life. These conflicting phenomena have forced a compromise in prior glass making apparatus.
The limitations imposed by erodable furnace wall materials can be substantially eliminated by the use of a crucible with nonerodible walls such as platinum (often alloyed with iridium or rhodium). Precision glass formulations are made this way but only small sized furnaces are feasible because of the cost of the platinum. Tungsten, rhenium and molybdenum are less expensive but oxidize at glass melting temperatures in the presence of oxygen, however, when submerged beneath liquid glass little or no oxidation or other chemical reaction takes place. Molybdenum is used routinely as an electrode material in electrically heated glass tanks. Graphite and silicon carbide are the other principal materials which are substantially unreactive with glass and are used in glass processing apparatus.
Platinum is often used to prevent erosion only at certain key areas in the furnace or in specialized apparatus. For example in U.S. Pat. No. 3,401,536, Apparatus for Melting and Processing Heat-Softenable Mineral Materials, issued Sept. 17, 1968, to Glaser, a chamber or crucible as well as a feeder are fashioned of an alloy of platinum or other material capable of withstanding the high temperature glass. In the apparatus of that patent a reservoir or holding chamber receives molten glass from a melter positioned above the crucible, with the melter receiving marbles of prefined glass from a hopper. The liquid glass passes a combination of baffles which reverse the direction of flow upwardly and over a baffle thus enhancing the removal of gases evolved from the glass.
U.S. Pat. No. 3,337,675, Manufacture of Glass, issued Aug. 22, 1967, to Descarsin, discloses another apparatus utilizing noble metal crucibles, such as platinum. Another furnace apparatus is shown and described in U.S. Pat. No. 3,358,066, Apparatus for Melting and Feeding Heat-Softenable materials, issued Dec. 21, 1967, to Tiede et al. This furnace is designed for producing elements of glass and is constructed of platinum with bushing tips being provided for passage of the glass therethrough. The apparatus employs a stirrer or impeller for mixing the glass to provide a homogeneous mixture. Other apparatus used in manufacturing glass are shown in U.S. Pat. No. 3,268,321, Apparatus for Forming Solid Glassware in a Carbon Die, issued Aug. 23, 1966, to Chapman and U.S. Pat. No. 4,155,731, Fiber Forming Bushing Construction, issued May 22, 1979, to Byrnes et al.
U.S. Pat. No. 3,320,095 issued June 21, 1963 to W. Weiss, et al discloses the use of a gas mixture of 80% argon 20% hydrogen in the processing of quartz to resist the outdiffusion of nitrogen. This mixture sometimes with somewhat less hydrogen has been found useful in the protection of tungsten or molybdenum when being used to melt glass. U.S. Pat. No. 2,871,411 issued Feb. 12, 1957 to W. Geffchen, et al discloses that platinum, tungsten and molybdenum do not react with glass when used as electrodes in heating glass by the Joule effect at frequencies between 500 and 1000 hertz. U.S. Pat. No. 3,015,191 issued Jan. 2, 1962 to P. Arbeit discloses the use of gas bubbling as a means of stimulating circulation in large glass furnaces but does not employ the bubbling technique for the direct mixing of the glass, which technique is very effective.
Copending U.S. patent application Ser. No. 6/370,347 filed on Apr. 21, 1982 and entitled Method and Apparatus for Making Fused Quartz and for Forming Glass Tubing, by the same inventor and owner as this application discloses a "drain valve" used to block the flow of glass from the first chamber to the second chamber while batch vacuum melting takes place. When that operation has been completed the valve heater is activated, the glass plug is melted from the transfer tube and the glass is drained into the second chamber. That "drain valve" is a device with on-off controls only. The present invention extends that art by introducing variable control and operating both the heating and cooling means concurrently and continuously. The result is continuous control of the rate of flow. It should be further noted that the same control system could be controlling fluid pressure. This is often important for fast response because the time constant of the flow control valve is relatively slow whereas the time constant and viscous response to pressure changes are almost instantaneous.
It is an object of the present invention to provide a new and improved method and apparatus for homogenizing viscous liquids and of separating bubbles therefrom.
It is still another object of the present invention to provide a new and improved method and apparatus using elevated pressure to suppress volatization and to force the partially fined glass through the homogenizer as well as other parts of the furnace.
It is still another object of the present invention to provide an improved system for controlling the flow and thermal extrusion of molten glass through the furnace.
It is still another object of the invention to provide method of salvaging heat energy from hot glass and using it to heat the glass constituents entering the heating zone of a furnace.
It is still another object of the present invention to provide a new and improved support system for a glass melting furnace.
It is still another object of the present invention to provide a new type of thermally controlled valve to control the rate of flow of viscous liquids.
It is still another object of the present invention to provide a new and improved glass cooling device for use in a forehearth of a glass furnace.
It is still another object of the present invention to provide a new and improved conditioning tank for use in the forehearth of a glass furnace.
It is still another object of the present invention to provide and new and improved extrusion nozzle to be used to produce cane, tubing or gobs of glass.